1. Field of the Invention
This invention relates to the field of liquid fertilizers prepared from wet-process phosphoric acid, and especially to an improved process for stabilizing and enhancing the color of wet-process phosphoric acid to produce a marketable product having color, clarity, and stability characteristics acceptable to the consumer both before and after conversion into liquid fertilizer. More particularly, it is concerned with such a process which involves controlled, sequential oxidation and reduction steps which unexpectedly produce a very desirable, green-colored stabilized acid from less desirable black acid.
2. Description of the Prior Art
For economic reasons, phosphoric acid used in the production of fertilizers, including liquids, has generally been obtained from phosphate rock by the wet process. This process includes the steps of acidulating the rock with sulfuric acid to precipitate calcium sulfate, solids removal by filtration, and concentration of the weak acid to a desired level by any one of a number of conventional evaporative techniques. The weak filter-grade acid is concentrated to different commercial grades or acid strengths ranging from 42% to 72% acid (P.sub.2 O.sub.5 basis) during such evaporation processes. The acid liquor is exposed to increasing temperatures during concentration, attaining a temperature between 370.degree. and 420.degree. F. at less than atmospheric pressures for the production of 68%-72% P.sub.2 O.sub.5 equivalent acid or superphosphoric acid (SPA). SPA contains the P.sub.2 O.sub.5 equivalent of approximately 100% H.sub.3 PO.sub.4 as well as varying amounts of polyphosphate. SPA is the preferred commercial acid for the manufacture of ammonium phosphate liquid fertilizer, and is typically reacted with anhydrous ammonia using a pipe reactor to form common liquid fertilizer grades. The acid liquor is exposed to high temperatures usually ranging from 450.degree. to 750.degree. F. generated by the heat of the reaction during conversion into liquid fertilizer. The high temperatures of concentration and conversion have the beneficial effect of promoting the formation of polyphosphates from orthophosphate, which tend to stabilize by sequestration of certain metallic impurities present in the acid. Unfortunatelly, other impurities including organic materials are destabilized or undesirably altered under the same conditions.
Although filtration removes most of the calcium sulfate solids during acidulation, the weak acid still contains a wide variety of chemical impurities originating from the rock that are solubilized and dispersed within the acid liquor. The principal metallic impurities consist of iron, aluminum, and magnesium oxides, salts, sulfates, and complexes; and the trace metallic impurities consist of similar compounds containing manganese, vanadium, chromium, zinc, cadmium, cobalt, and uranium. Acid-soluble and acid-insoluble organic impurities composed of colloidal carbon, humic acids and aldehydes, alkanes, keratins, and bitumins are also present in the acids. Other impurities include residual sodium, potassium, and calcium salts, sulfates, and silicates, including fluoride and chloride salts and complexes.
The insoluble and soluble organic material present in the weak phosphoric acid produced by acidulation form a matrix that enhances and stabilizes the foam of the acid liquor during concentration. Foaming occurs when gaseous water and residual carbon dioxide percolate through the acid during evaporation and are entrapped by the organic impurities in the acid liquor. The stabilized foam reduces the rate of evaporation and flow rate through the evaporators, and some entrained foam passes out of the evaporators with the vapor phase. Losses of P.sub.2 O.sub.5 contained within the entrained foam are equivalent to 1 to 5 weight percent of the 68% SPA product. A variety of carbon-base antifoam products are added to the weak acid during concentration to destabilize the foam and retard its formation. The organic impurities, through foaming, contribute significantly to the costs associated with the production of superphosphoric acid by increasing the loss of P.sub.2 O.sub.5 and necessitating the addition of antifoam products.
The organic impurities, including any residual antifoam product, char or form an opaque carbon floc when exposed to the high temperature, reducing environment present during concentration of the acid. The carbon floc forms a viscous matrix that increases the viscosity of the acid and hinders the removal of other undesirable metallic impurities. Such metallic impurities are typically removed from superphosphoric acid by filtration and the carbon floc binds the filter cloth and filter media, resulting in unacceptable and uneconomical pressure drop across the filter and excessively long filtration times.
The carbon floc carries over during conversion of the concentrated acid into liquid fertilizer and imparts an objectionable black or dark color to the product. Furthermore, the carbon floc reduces the clarity of the liquid fertilizer and obscures the presence of metal phosphate precipitates that may clog spray nozzles or settle out into insoluble sludge during storage. As little as 0.2 weight percent of carbonaceous material in the acid is sufficient to discolor or darken the final liquid fertilizer and greatly reduce its marketability.
The reducing conditions present during acidulation and concentration also tend to promote the formation of lower-oxidation-state metallic complexes such as ferrous phosphate or manganese II phosphate, rather than ferric phosphate or manganese III phosphate. Concentrated wet-process phosphoric acid typically contains a higher percentage of lower-oxidation-state transition metals compared to higher-oxidation-state transition metals. Many of the lower-oxidation-state transition metals, including Fe.sup.+2, form destabilizing phosphate complexes that promote sludge formation in liquid fertilizer during storage. Metallic impurities containing divalent cations such as ferrous iron and magnesium are difficult to sequester and are more favorably sequestered by tripolyphosphate rather than pyrophosphate. SPA typically contains more pyrophosphate than tripolyphosphate. Thus ferric impurities are more effectively sequestered in SPA and stay in solution longer, as compared to impurities containing divalent cations. A number of insoluble magnesium ammonium phosphate compounds may form during storage in a 10-34-0 solution fertilizer produced from phosphoric acid contaminated with metallic impurities and fluorine impurities. The most common precipitates are compounds similar to MgAl(NH.sub.4).sub.5 F.sub.2 (P.sub.2 O.sub.7).sub.2.6H.sub.2 O. As much as 20-25 weight percent sludge may form in ammonium and potassium polyphosphate liquid fertilizers contaminated by as little as 0.4% ferrous iron. These insoluble precipitates can clog spray nozzle orifices as well as pumps and transfer lines and build up in storage tanks and tank cars. Ferric phosphates, however, do not as readily form insoluble sludges.
Transition metals such as iron, manganese, and vanadium also form intensely colored phosphate complexes, whose color is often dependent upon the oxidation state of the metallic species. Colored metallic complexes that darken or blacken the concentrated phosphoric acid either before or after conversion into liquid fertilizer greatly reduce the marketability of the acid, and have a deleterious effect similar to carbon floc upon the salability of superphosphoric acid.
Historically, black-colored, opaque superphosphoric acid has been an extremely difficult product to sell for the manufacture of liquid fertilizer. Historical data indicate that liquid fertilizer manufactured from black superphosphoric acid tends to clog spray nozzles and settle out into insoluble sludge when stored for lengthy periods of time. Research has indicated that the sludge that forms in liquid fertilizer manufactured from black acid is directly related to magnesium, aluminum, and iron impurities present in the acid. Moreover, it is believed that sludge formation may be catalyzed by trace amounts of lower oxidation-state transition metal complexes containing such metals as manganese, vanadium, nickel, and zinc. These minor complexes may promote the formation of metal phosphate sludge from iron, magnesium, and aluminum impurities that are present in significantly higher concentrations in the acid as compared with the minor complexes. Thus, the destabilizing properties of the black acid are primarily caused by the major and minor lower oxidation-state metallic impurities and are not wholly related to the black color or carbon content of the acid. Removal of most of these metallic contaminants or neutralizing their destabilizing properties by raising their oxidation state would therefore produce a serviceable product with good stability and solubility characteristics, even though the acid remains blackcolored.
However, the organic carbonaceous material obscures the presence in the acid of excessive amounts of finely divided solid metallic impurities. The impurities obscured by the black color of the acid may further contribute to a buildup of scale within the pipe reactors used to convert the SPA and ammonia into black-colored 10-34-0 liquid fertilizer. High levels of magnesium and aluminum carried over from the acid can react to form large amounts of additional insoluble sludge in 10-34-0 typically stored at warm temperatures during the summer season. Thus, most consumers associate the black color with an unstable and inferior product, and prefer to purchase a green-colored clarified phosphoric acid typically produced from calcined phosphate rock. The carbonaceous material present in black acid also impedes the removal of metallic impurities from the acid by filtration and totally interferes with the removal by solvent extraction of certain valuable metallic impurities including uranium. Organic compounds such as humic acids form stable emulsions with the solvents used to extract uranium, and these emulsions build up in continuous countercurrent extractions systems and lower the efficiency of the uranium extraction.
Accordingly, there are a number of very good reasons to decarbonize phosphoric acid from which the metallic impurities have been removed. As a consequence, workers in the art have devoted considerable time and effort to developing economical, commercially viable clarification procedures.
A review of the literature reveals a number of attempts to solve the problem by physical methods. Such include absorption on activated carbon or perlite; flocculation with tall oil pitch, sulfonated polystyrene, polyacrylamide, phendol formaldehyde or fatty acid; simple organic solvent extraction with a hydrocarbon such as kerosene; precipitation with H.sub.3 BO.sub.3 or borate, or silicate; filtration through colloidal clay, bleaching clay or bentonite and granular carbon; dilution and filtration; and dilution and clarification with rinsate from an aluminum polishing process.
These physical methods successfully decarbonized weak phosphoric acid or SPA diluted to a concentration similar to weak acid. However, the clarified acid produced by the above physical methods turns black again or darkens when concentrated to SPA. Therefore, many investigators have concluded that a successful decarbonization process must remove carbon from SPA rather than from weak 54%, 42%, or filter-grade acid. The above physical processes do not effectively remove carbonaceous material from SPA.
Several chemical processes during strong oxidizing agents may be used to decarbonize SPA. Oxidation of the carbon to CO.sub.2 appears to be the most effective process. The following oxidizing reagents are also described in the literature for decarbonization of phosphoric acid: nitric acid and ammonium nitrate; iodate; chromate; permanganate and peroxide; and oxygen and air. Iodate, chromate, permanganate, and peroxide are effective oxidants but appear to increase production costs by more than is acceptable for commercial purposes. Chromate and iodate oxidation would also leave toxic by-products in the acid that could not be easily or inexpensively removed. Oxygen and air oxidation would require significant capital investment for pressurized corrosion-resistant reactors. Ammonium nitrate and nitric acid are less expensive relative to the other oxidants described in the literature.
Defensive Publication No. T892,005 to Scheib, 8920.G.1210, published Nov. 23, 1971, discloses a process wherein black wet-process phosphoric acid and mixtures of wet-process and electric furnance acid are preheated to 250.degree. F. and mixed with around 0.1 to 2 weight percent ammonium nitrate. Scheib teaches that the ammonium nitrate should be added subsequent to heating to preclude loss of oxidizing potential prior to reaching the most effective temperature for oxidation and to minimize undesirable foam production and accumulation. The minimum recommended oxidation temperature is 250.degree. F., and the maximum temperature is around 400.degree. F. Scheib cautioned that further decomposition of dissolved organic material and return of the black color could occur if the clarified acid was raised to a temperature higher than 400.degree. F. during the oxidation process or in subsequent processing.
Typical processing temperatures may not exceed 400.degree. F. during the concentration of weak acid up to SPA in an evaporator. However, SPA is often exposed to much higher temperatures during conversion into 10-34-0. Indeed, the temperature of the SPA may exceed 600.degree. F. during the reaction with anhydrous ammonia in a pipe reactor and immediately thereafter. Instantaneous temperatures within localized microenvironments of the reacting fluid may be even higher. The Scheib process may not remove enough carbon from the acid to prevent blackening of the product upon conversion into 10-34-0 within the pipe reactor. No provisions are made in the Scheib process to stabilize or remove dark-colored metallic complexes that could darken the acid during subsequent processing.
The process described in Canadian Pat. No. 955,033 to Moore is specifically designed to produce light-colored, decarboned SPA that will not blacken when reacted with aqueous ammonia solution using a continuously stirred tank reactor (CSTR) during production of 10-34-0. Nitric acid, ammonium nitrate, and sodium perchlorate are the preferred reagents for this oxidation process. Typical reaction mixtures contain between 0.75 and 10 equivalents (based upon oxygen) of oxidant per mole of carbon present in the SPA, and the oxidant may be added to the SPA at ambient temperatures.
The oxidation reaction is carried out at reaction temperatures between 250.degree. and 650.degree. F., preferably using a single vented CSTR. Best results were obtained by mixing 40 lb. 67% HNO.sub.3 per ton of SPA (68% basis) or 50 lb. NH.sub.4 NO.sub.3 per ton SPA (68% basis) and reacting the mixture in a CSTR at 440.degree. F. for around 1-5 minutes. Evolved gases consisted of a mixture of water vapor, carbon dioxide, nitrogen oxides, and nitrogen. The resulting oxidized SPA is described by Moore as containing approximately the same polyphosphate content as black acid.
The decolorized SPA was reacted with aqueous ammonia around 140.degree. F. in a CSTR to produce aqueous yellow-or tan-colored 10-34-0 or other similar ammonium phosphate liquid fertilizers, rather than a desirable green-colored product. The single-stage decarbonization reaction described by the Moore process therefore does not provide for enhancement of the color of the decarbonized SPA. Further, no attempt is made in the Moore process to adjust or stabilize the oxidation state of the various colored transition metal complexes present in the acid.
U.S. Pat. No. 4,420,321 to Wilson is significantly different from the processes developed by both Moore and Scheib in that carbon floc is sought to be removed from SPA during conversion of the acid into liquid ammonium phosphate fertilizer using a pipe reactor. An oxidant such as nitric acid is added to SPA and anhydrous ammonia in the pipe reactor where reaction conditions typically generate temperatures ranging from 500.degree. to 700.degree. F. and pressures exceeding 400 psig. Around 5.26 tons HNO.sub.3 are postulated to decarbonize the 1 ton of carbonaceous floc typically found in 250.4 tons 70% SPA (P.sub.2 O.sub.5 basis). Wilson asserts that liquid fertilizer produced by the process should be clear and colorless and the evolved by-product gases should be clear and odorless.
Wilson did not report the results of an actual test of this process in which anhydrous ammonia, SPA, and HNO.sub.3 were reacted together in a pipe reactor. The simulated test decarbonized 10-34-0 ammonium polyphosphate liquid fertilizer with HNO.sub.3 using a pressurized and heated pipe, attaining a reaction temperature of 590.degree. F. and pressure of 400 psig. Results of the simulated test indicated that decarbonization did not occur until after 49 minutes of continuous processing. The residence time during actual liquid fertilizer pipe reactor conditions is relatively short, on the order of seconds, and it is believed that this may be insufficient time to satisfactorily decarbonize the SPA using the Wilson process. In addition, no provision is made in the process to enhance the color or stability of the SPA or liquid fertilizer.
Furthermore, the Wilson process could not easily be implemented in the field. Clear, green-colored SPA is the favored acid of commerce and is transported from the manufacturer to remote locations for on-site conversion into liquid fertilizer. Black SPA is not generally accepted by those who formulate liquid fertilizer. A formulator would be required to modify existing equipment or purchase additional equipment to successfully utilize the Wilson process. At the very least, HNO.sub.3 is a corrosive and hazardous material that is not easily transported, stored, and utilized by those who formulate liquid fertilizer.
Finally, the Wilson process is not versatile. In the process the optimum temperatures for decarbonization are generated by the heat of formation of ammonium phosphate, and the end product is ammonium polyphosphate fertilizer. The Wilson process is specifically designed to produce liquid fertilizer, not SPA. The heat generated during the reaction of ammonia with phosphoric acid is used during the process to accelerate the reactions between oxidant and carbonaceous material present in the mixture, and the product of the proces is ammonium polyphosphate, not phosphoric acid. The process cannot be used to purify, stabilize and decarbonize weak phosphoric acid; i.e., concentrations of phosphoric acid that are less than 68% superphosphoric acids. Thus the Wilson process cannot be used to enhance the production of SPA.
Decarbonization of wet-process acid (WPA) or SPA by oxidation using oxidants containing a nitrate group such as ammonium nitrate or nitric acid has not yet been commercially implemented, primarily because of poor product quality and air pollution problems. It has not been possible for those skilled in the art to produce a desirably colored green acid, and especially, a green-colored WPA that maintains the desirable color without darkening during subsequent processing into more concentrated acid or liquid fertilizer, by the use of a simple, single-stage oxidation process. The concentration of NO.sub.x off-gases produced by previous oxidation processes was sufficiently high to require the use of complicated and expensive pollution control systems. Such problems have precluded the use of existing nitrate oxidation processes for the decarbonization of phosphoric acid.